1. Field of the Invention
The present invention relates generally to printing presses and more particularly to a device and method for measuring or controlling web tension in a printing press.
2. Background Information
Web printing presses print a continuous web of material, such as paper. In a printing unit of the printing press the continuous web is printed with an image, and then the web is folded and cut into signatures. The process of printing an image to the web, as well as folding of the web, creates tension in the web. Correct tension should be maintained in the web during the process of printing and folding.
One way to measure the correct tension is through a tension transducer device. However, existing tension transducers are analog devices that require frequent calibration and are subject to damage if overloaded. Also, the bandwidth of existing tension transducers is insufficient for some applications. Furthermore, existing tension transducer designs consist of strain gauge assemblies mounted in the end of tension rolls. The assemblies make some applications, e.g., measuring web tension on a driven roll or on a liquid cooled roll, difficult, expensive, or impossible.
An object of the present invention is to stabilize a roll by counteracting the force produced on the roll by the web tension. An additional or alternative object of the present invention is to provide a simpler device to produce web tension information from rolls. Yet another additional or alternative object of the present invention is to provide a device that can act as both a compensator and a tension measuring device in a web span.
The present invention provides a device for measurement of web tension comprising:
a roller for contacting a web of material, the roller having a rotational axis, the axis being movable in a first direction by the web;
a counteracting device connected to the roller, the counteracting device for forcing the axis of the roller in a second direction opposite the first direction; and
a controller connected to the counteracting device for measuring the web tension.
With the roller and counteracting device of the present invention, web tension information may be easily and efficiently obtained. Moreover, the controller allows the present invention to function as both a ribbon compensator and a tension measuring device in a given web span.
The movable roller may comprise a liquid cooled roll. Advantageously, the liquid cooled roll improves performance of web printing.
Preferably, the counteracting device is a motor. The motor provides the advantage of allowing the present invention to operate at higher tensions with less likelihood of damaging or jamming.
The controller, preferably, is a solid state device. The use of a solid state controller eliminates frequent calibration of the present invention.
The web tension measurement device may include a plurality of lever arms, a pivot shaft, a plurality of drive sprockets, a belt, a counteracting device shaft, and/or a roll shaft, with each device mechanically linked to the counteracting device.
The present invention also includes a method for measuring tension in a web comprising the steps of:
running a web over a roller having an axis, the axis of the roller being movable in a first direction;
counteracting the movement of the axis in a second direction opposite the first direction; and
measuring a counteracting force or a variable so as to determine a web tension.
The roller may remain stationary in the method for measuring tension in the web. Advantageously, maintaining the roller in a stationary position simplifies obtaining web tension information.
In a preferred embodiment, the roller may be rotatable in a lever or lever arm about a pivot axis, where a web compensator algorithm determines the desired position of the roller. The rotation about a pivot axis provides the advantage of allowing the roller to perform the tension measurement function as well as functioning as a web or ribbon compensator.